On the Cyclopolymerization of 1,6-Heptadienes, and their Role as Poly(methylenecycloalkane)s in Stereoengineering and Block Copolymers

The research presented herein, addresses key issues of homogeneous Group 4 single-site coordination polymerization (CP) catalysts for the production of polyolefins and polyolefin-like materials. Specifically, this research moves beyond the `one-catalyst one-material' paradigm to afford an array of amorphous polyolefin materials with high Tg from a single monomer. The multitude of microstructurally distinct materials available from a single starting olefin is attributed to stereoengineering: a technique, which reduces stereoblock length in a highly controlled fashion while retaining regioselectivity. The precatalysts employed in this work are previously reported Group 4 CS-symmetric or C1-symmetric pentamethylmonocyclopentadienyl amidinate complexes with the general formula {(η5-C5R5)M[N(R1)C(R2)N(R3)]-(Me2)} (M = Zr, Hf, R = alkyl, Me = methyl), which are activated by cocatalysts such as N,N-dimethylanilinium tetrakis(pentafluorophenyl)-borate ([PhNMe2H][B(C6F5)4]). Living CP of 1,6-heptadiene and stereoengineering of the subsequent poly(methylenecycloalkane)s with the above complexes reveal a variety of stereochemically controlled, yet amorphous, poly(methylene-1,3-cyclohexane) (PMCH) materials with Tg values as high as 101 °C. Similar polymerization techniques have been applied, for the first time with Sita group complexes, towards the CP of the heteroatom-olefins such as diallyldimethylsilane (DAS). The controlled CP and stereoengineering of DAS resulted in amorphous poly(3,5-methylene-1,1-dimethyl-1-silacyclohexane) materials with Tg values as high as 127 °C. The living character and tunable stereoblock lengths of PMCH provided the opportunity to explore the high Tg polyolefin as the `hard' domain (A segment) in pure polyolefin AB block copolymers, BCPs. Specifically, amorphous AB diblock copolymers were synthesized using poly(1-hexene) as the `soft' B block to afford a series of microphase-separated morphologies without the deleterious effects of crystallization. Microphase-separated morphologies for were also observed for ABA triblock copolymers using atactic polypropylene as the `soft' segment (B block) and primary component. The latter BCPs were found to exhibit thermoplastic elastomeric properties. The work described in this document provides a foundation for the further expansion of the currently-limited pool of monomers to include heteroatom-olefins for CP with the aforementioned Group 4 transition metal complexes. Moreover, the formation of well-defined pure polyolefin block copolymers serve as an important contribution to the development of new polyolefin architectures

Effects of digital chatbot on gender attitudes and exposure to intimate partner violence among young women in South Africa

Background South Africa has among the highest rates of intimate partner violence (IPV) globally, with young women at heightened risk due to inequitable gender roles, limited relationship skills, and inadequate social support. Despite an urgent need for violence prevention in low- and middle-income settings, most efficacious approaches are time-intensive and costly to deliver. Digital, interactive chatbots may help young women navigate safer relationships and develop healthier gender beliefs and skills Methods Young women (18–24 years old) across South Africa were recruited via Facebook for participation in an individually randomised controlled trial (n = 19,643) during the period of June 2021-September 2021. Users were randomly allocated, using a pipeline algorithm, to one of four trial arms: Pure Control (PC) had no user engagement outside of study measures; Attention Treatment (T0) provided didactic information about sexual health through a text-based chatbot; Gamified Treatment (T1) was a behaviourally-informed gamified text-based chatbot; Narrative Treatment (T2) was a behaviourally-informed drama delivered through pre-recorded voice notes. All chatbots were delivered in WhatsApp, through which users were invited to complete brief “quizzes” comprising adapted versions of validated scales. Primary outcomes were short-form adaptations of scales for gender attitudes (Gender Relations Scale) and past-month IPV (WHO Multi-country Study Instrument). Secondary outcomes were identification of unhealthy relationship behaviours (Intimate Partner Violence Attitudes Scale) and brief screener for depressive symptoms (Patient Health Questionnaire). A direct chat link to a trained counsellor was a safety measure (accessed by 4.5% of the sample). We estimated treatment effects using ordinary least squares and heteroskedasticity robust standard errors Findings The trial retained 11,630 (59.2%) to the primary endpoint of gender attitudes. Compared to control, all treatments led to moderate and significant changes in attitudes towards greater gender equity (Cohen’s D = 0.10, 0.29, 0.20 for T0, T1, and T2, respectively). The gamified chatbot (T1) had modest but significant effects on IPV: 56% of young women reported past-month IPV, compared to 62% among those without treatment (marginal effects = -0.07, 95%CI = -0.09to-0.05). The narrative treatment (T2) had no effect on IPV exposure. T1 increased identification of unhealthy relationship behaviours at a moderate and significant level (Cohen’s D = 0.25). Neither T1 nor T2 had a measurable effect on depressive symptoms as measured by the brief screener. Interpretation: A behaviourally-informed, gamified chatbot increased gender equitable attitudes and was protective for IPV exposure among young women in South Africa. These effects, while modest in magnitude, could represent a meaningful impact given potential to scale the low-cost intervention

Regio- and Stereospecific Cyclopolymerization of Bis(2-propenyl)diorganosilanes and the Two-State Stereoengineering of 3,5-<i>cis</i>,<i>isotactic</i> Poly(3,5-methylene-1-silacyclohexane)s

Transition-metal-mediated coordination cyclopolymerization of bis­(2-propenyl)­dimethylsilane (<b>1a</b>) using the <i>C</i>₁-symmetric, group 4 metal preinitiator, (η⁵-C₅Me₅)­Zr­(Me)₂[N­(Et)­C­(Me)­N­(^tBu)] (<b>I</b>), in combination with 1 equiv of the borate coinitiator, [PhNHMe₂]­[B­(C₆F₅)₄] (<b>II</b>), proceeds in a regio- and stereospecific manner to provide highly stereoregular 3,5-<i>cis</i>,<i>isotactic</i> poly­(3,5-methylene-1,1-dimethyl-1-silacyclohexane) (<b>2a</b>). Successful stereoengineering of <b>2a</b> to eliminate undesirable crystallinity while preserving a high <i>T</i>_g value of >120 °C was subsequently accomplished by employing a “two-state” propagation system that uniquely produces an isotactic stereoblock microstructure of decreasing stereoblock length with decreasing percent level of “activation” of <b>I</b> with <b>II</b>. The controlled character of cyclopolymerization of <b>1a</b> using the less sterically encumbered preinitiator, (η⁵-C₅Me₅)­Hf­(Me)₂[N­(Et)­C­(Me)­N­(Et)] (<b>III</b>), and 1 equiv of <b>II</b> was used to prepare well-defined poly­(1-hexene)-<i>b</i>-poly­(3,5-methylene-1-silacyclohexane) block copolymers through sequential monomer additions

Stereoengineering of Poly(1,3-methylenecyclohexane) via Two-State Living Coordination Polymerization of 1,6-Heptadiene

External control over the rate of dynamic methyl group exchange between configurationally stable active species and configurationally unstable dormant species with respect to chain-growth propagation within a highly stereoselective and regiospecific living coordination polymerization of 1,6-heptadiene has been used to generate a spectrum of different physical forms of poly­(1,3-methylenecyclohexane) (PMCH) in which the stereochemical microstructure has been systematically varied between two limiting forms. The application of this strategy to manipulate the bulk properties of PMCH and the solid-state microphase behavior of well-defined PMCH-<i>b</i>-poly­(1-hexene) block copolymers is further demonstrated

<i>De Novo</i> Design of a New Class of “Hard–Soft” Amorphous, Microphase-Separated, Polyolefin Block Copolymer Thermoplastic Elastomers

Sequential cyclic/linear/cyclic living coordination polymerization of 1,6-heptadiene (HPD), propene, and HPD, respectively, employing the well-defined and soluble group 4 transition-metal initiator, {(η⁵-C₅Me₅)­Hf­(Me)­[N­(Et)­C­(Me)­N­(Et)]}­[B­(C₆F₅)₄], provides the stereoirregular, amorphous poly­(1,3-methylenecyclohexane)-<i>b</i>-atactic polypropene-<i>b</i>-poly­(1,3-methylenecyclohexane) (PMCH-<i>b-</i>aPP-<i>b</i>-PMCH) polyolefin triblock copolymer (<b>I</b>) in excellent yield. By varying the weight fraction of the end group, minor component “hard” PMCH block domains, <i>f</i>_PMCH, relative to that of the midblock “soft” aPP domain, three different compositional grades of these polyolefin block copolymers, <b>Ia–c</b>, were prepared and shown by AFM and TEM to adopt microphase-separated morphologies in the solid state, with spherical and cylindrical morphologies being observed for <i>f</i>_PMCH = 0.09 (<b>Ia</b>) and 0.23 (<b>Ic</b>), respectively, and a third more complex morphology being observed for <b>Ib</b> (<i>f</i>_PMCH = 0.17). Tensile testing of <b>Ia–c</b> served to establish these materials as a new structural class of polyolefin thermoplastic elastomers, with <b>Ia</b> being associated with superior elastic recovery (94 ± 1%) after each of several stress–strain cycles

Advanced Approaches For Quantitative Characterization Of Thermal Transport Properties In Soft Materials Using Thin, Conformable Resistive Sensors

Noninvasive methods for precise characterization of the thermal properties of soft biological tissues such as the skin can yield vital details about physiological health status including at critical intervals during recovery following skin injury. Here, we introduce quantitative measurement and characterization methods that allow rapid, accurate determination of the thermal conductivity of soft materials using thin, skin-like resistive sensor platforms. Systematic evaluations of skin at eight different locations and of six different synthetic skin-mimicking materials across sensor sizes, measurement times, and surface geometries (planar, highly curvilinear) validate simple scaling laws for data interpretation and parameter extraction. As an example of the possibilities, changes in the thermal properties of skin (volar forearm) can be monitored during recovery from exposure to ultraviolet radiation (sunburn) and to stressors associated with localized heating and cooling. More generally, the results described here facilitate rapid, non-invasive thermal measurements on broad classes of biological and non-biological soft materials

Natural Wax For Transient Electronics

Emerging classes of bioresorbable electronic materials serve as the basis for active biomedical implants that are capable of providing sensing, monitoring, stimulating, and other forms of function over an operating period matched to biological processes such as wound healing. These platforms are of interest because subsequent dissolution, enzymatic degradation, and/or bioresorption can eliminate the need for surgical extraction. This report introduces natural wax materials as long-lived, hydrophobic encapsulation layers for such systems, where biodegradation eventually occurs by chain scission. Studies of wax stability as an encapsulation material demonstrate the ability to retain operation of underlying biodegradable electronic systems for durations between a few days to a few weeks during complete immersion in aqueous solutions in ex-vivo physiological conditions. Electrically conductive composites result from the addition of tungsten micro/nanoparticles, as a conductive, printable paste with similar lifetimes. Demonstrations of these materials in partially biodegradable wireless light-emitting diodes and near-field communication circuits illustrate their use in functional bioresorbable electronic systems. Investigations in animal models reveal no signs of toxicity or other adverse biological responses

Polymeric Materials for Hemostatic Wound Healing

Hemorrhage is one of the greatest threats to life on the battlefield, accounting for 50% of total deaths. Nearly 86% of combat deaths occur within the first 30 min after wounding. While external wound injuries can be treated mostly using visual inspection, abdominal or internal hemorrhages are more challenging to treat with regular hemostatic dressings because of deep wounds and points of injury that cannot be located properly. The need to treat trauma wounds from limbs, abdomen, liver, stomach, colon, spleen, arterial, venous, and/or parenchymal hemorrhage accompanied by severe bleeding requires an immediate solution that the first responders can apply to reduce rapid exsanguinations from external wounds, including in military operations. This necessitates the development of a unique, easy-to-use, FDA-approved hemostatic treatment that can deliver the agent in less than 30 s and stop bleeding within the first 1 to 2 min at the point of injury without application of manual pressure on the wounded area